Question about loading with an SUT

I know this isn't going to do any good, but I'll advise you not to worry about it. 

+1

From my understanding, when using a SUT, gain tends to play a more critical role than loading.

That said, if you're already getting great sound from both cartridges with the same SUT, I wouldn't worry about it as mentioned by others. By the way, you didn’t mention which phono stage you’re pairing with your setup.

I have a DL-103R feeding into a Cinemag 3440 with switchable taps for 1:18 (150 ohm) or 1:36 (37.5 ohm). I use 1:18 and it sounds lovely. I also have a Rothwell MCL with 1:20 (100 ohm) and I honestly can't hear a difference between these two SUTs. I've switched back and forth many times and settled on the Cinemag. 

The 1:36 ratio with the Denon cartridge gives you an impedance that the Denon cannot drive efficiently. About 50% of its signal voltage output will be lost to ground. So doubling the turns ratio from 1:18 to 1:36 gives you nothing in terms of added gain. In addition at the higher ratio the treble may roll off due to the effect of capacitance. My point being that with a SUT, impedance as affected by turns ratio is every bit as important as voltage gain, and it’s usually a trade off.

I have a Zesto SUT with adjustable ratios and loadings going into a EAT eglo petite also with adjustable gain and loadings for an Aphelion2 cart (and previously a Soundsmith Zephyr MIMC*). In both cases the recommended loadings sounded best to my ears. Differences between settings are extraordinarily subtle to zero, so only major mismatches made a clear difference. In your case, the recommended and available values are very close, so as others have said, don't worry about it. 

I just realized the 103R has an internal resistance of 14 ohms, as compared to the standard DL103 with R = ~30 ohms. Therefore I have to temper my earlier advice. Setting the SUT ratio at 1:36 would still be suboptimal but not as bad as I indicated. Ideally you want the output impedance, in this case the internal R, to be at least one tenth of the input impedance of in this case the phono stage. A higher ratio is fine. At lower ratios more and more of the cartridge output is lost to ground. When the two impedances are equal, 50% of the signal is lost.

I’m wondering if there would really be much of a difference if I had an SUT with the exact ratio and loading that is specified rather than "getting close?"

@adam8179 Transformers will ’ring’ (distort) if insufficiently loaded and will roll off if loaded too much. So the answer is yes. You can hear it and its easily measured.

The correct loading value varies with the source impedance (the cartridge).

Transformers transform impedance; they do not isolate impedance, so the output impedance of the transformer varies with the source impedance.

Don’t worry about what is ’ideal’ for any LOMC cartridge- loading such cartridges for sound quality is a myth. Your main concern is making sure the transformer isn’t modifying the sound by being incorrectly loaded (although all SUTs make distortion and limit bandwidth even if loaded correctly).

This is tricky since the capacitance of the tonearm cable and that of the cable between the transformer and phono section plays a role. Quite literally this should be done with a square wave generator, an oscilloscope and a variable resistance. With the cartridge in place the square wave (of very low amplitude) is applied to the input side of the transformer and the output observed on the ’scope. The variable resistance is placed across the output (which is connected to the phono section) and varied until the ’scope shows something that looks as close to the square wave as possible. If this does not make sense to you find a technician to do it for you.

This value of resistance is called ’critical damping’.

Obviously this is tricky (in most cases will have to be done by a technician) and is one reason I prefer using a phono section that does not need an SUT; one competently designed so I don’t have to worry about ’cartridge loading’ (which is for the benefit of the phono section, not the cartridge); just plug and play.

As others have well pointed out, you should never stress about differences like 18x versus 20x. It truly doesn’t matter. If you swap from one brand/model of SUT to another, the inherent sonic differnces between those SUTs - due to materials choices, core size, lamination quality, wire windings, etc - will absolutley swamp out anything caused by small differences to loading and/or step-up ratios.

When playing with SUTs, what you need to get a sense of is the "bounds" you’re working in:

• If you load a cartidge down too heavily, you’ll start to lose increasingly more than 1dB of signal level, due to your phono stage’s input being on the "wrong side" of a voltage divider. This is a problem when you use too high of a SUT ration for a cartridge based on its coils’ DC impedance. You would’t want to use a 40x SUT with 38 ohm coils, beacause you will be losing 6dB of signal! This can also cause frequency response aberrations, besides the bulk signal loss.
• You can calculate out the losses, and you probably want to stay lower than 2dB loss in all cases. You’re fine with what you’re using here - well under 1dB of losses.The "load your cartridge at 10x its coil impedance" isn’t a bad rough guide. You can always go higher, no problem. Going lower, I find 5x - 6x to be the cutoff - lower than this is almost always bad.
• If you choose too high a SUT ratio - even if the loading works out OK and the signal losses are acceptable - you run the risk of encroaching overload margins on your phono stage. At first this can cause the sound to seem "dynamic, punchy" but as you push it the sound can become harsh, gritty. As a rule you need to keep the signal hitting your phono stage input under 10mV as calculated by the cartrdge’s rated output level. E.g. a Shelter rated at 0.5mV with a 20x SUT = 10mV (it will actually be a little less due to loading losses described above). It’s generally best to target 5mV, but up to 7.5mV is fine.
• Avoid cartridges with "inefficient" magnetic generators like Benz Ruby / Gullwing / LPS (Ebony / Wood / Glider series is fine), Dynavector Karat 17D, there are a few others. These cartridges have high coil DC resistances paired with relatively low output levels - they are NOT a good match to SUTs or "transimpedance" head-amps. The ruby plate armature (as opposed to iron / permendur) is at fault here. Ortofon has started using some non-magnetic armature materials in its top cartidges, but thy reign it in (hybrid) just enough to still allow use with carefully chosen SUT ratios.
• "Transimpedance" head-amps are the active analogue to a SUT (passive) - sort of. The loading (as close to 0 ohms as possible) is really weird and totally different. You can buy a Hagerman Piccolo Zero for cheap ($270), and it’s a lot of fun. Its musicality competes with much more expensive SUTs, at the expense of a higher noise floor. It’s almost silly how well this device matches with certain Ortofon SPUs (Classic GM Mk II spherical; Meister Silver) - this has actually been my favorite combo of the past couple months.

You left out “head amps” aka “pre-preamps” which have been around since the early 70s. These devices are active gain stages that boost cartridge output voltage ahead of the phono stage. They do not correct for RIAA. Sutherland and Hagerman make good ones but there are many other choices. When MC cartridges hit the US market in the 70s, most audiophiles owned only MM phono stages with insufficient gain to accommodate the MCs. Pre-preamps were the cure for that, even before there were many SUT choices. The good news is that a prepreamp will work with any MC, regardless of its internal resistance. So there are 3 viable options.

@chipcalzone , I'm using a Declare Z3 phonostage.

Thanks for all your responses. @atmasphere , this procedure is beyond my capabilities and leads me to consider a head amp.  In fact, many of the responses seem to be leading me in that direction.  I'm also good with the "don't worry about it" camp since I'm quite happy with the sound I'm getting.  Perhaps a Hagerman Piccolo is in my future but I think from this discussion, I'm going to rule out a different SUT since it seems they need to be tailored very specifically to the cart/tonearm cable and I don't want to have a different SUT for every cart.

A high gain phono stage can be an MM stage with an additional voltage gain stage tacked on to the input end. Such a topology can therefore handle both low and high output cartridges. Alternatively a high gain phono may simply incorporate a very high gain but unitary input voltage gain circuit.

I use five different carts and use one SUT currently wired at 1:10.

Perhaps a Hagerman Piccolo is in my future but I think from this discussion, I’m going to rule out a different SUT since it seems they need to be tailored very specifically to the cart/tonearm cable and I don’t want to have a different SUT for every cart.

It doesn’t need to be tailored, really. Any SUT in the range of 10x - 24x can be a pretty good general-purpose SUT for most low-ish to low-medium-ish MC cartridges (0.2mV - 0.6mV). Many SUTs also offer 2 taps, which is usually enough to cover "most" needs in an MC cartridge collection. Some even offer 3 or more taps - like the EAR MC-3 and MC-4 - these SUTs are extremely flexible, and sound great with almost any cartridge (excluding the high-output MCs, which are weird anyways).

If you do purchase a Piccolo (I’m a big fan of Hagerman phono gear in general), take note there are 2 versions: the Piccolo MC and Piccolo ZERO. The MC is a more traditional JFET-based "voltage mode" amplifier, with multiple gain levels and load settings. The ZERO is a "current mode" amplifier (a.k.a. transimpedance) that has "some" of the same considerations as a SUT - it won’t work well with cartridges that have high coil DC ohms. However, the Zero is spectacular on cartridges with very low DC ohms paired to efficient magnetic generators - like Ortofon SPUs.The Zero also offers 4 sensitivity levels (internal jumpers) for matching to a specific cartridge. I have a collection of SUTs, phono stages, and cartridges - and I love my Zero! It’s a big middle finger to the rest of the industry that Jim H. sells this for $270.

The Piccolo MC is also quite good by the way - I find Jim's JFET head-amps to be more "musical" than other JFET stages, at the expense of higher noise floor - a tradeoff that is acceptable to me. He does use very cheap SMPS units, in order to hit such low price points. There may be some gain in 3rd party linear PSU's. 

Adam, I’ve got two oscilloscopes, an audio signal generator, and etc. I’ve built and modified my own equipment, which includes Atmasphere amps and preamp. Even I would find Atmosphere’s suggested procedure daunting. There’s nothing wrong with following the few simple rules of impedance matching ( if you know and understand them and can do the simple math) and then using your ears. Down the road you might compare the results to the various kinds of active gain stages and see what you like best.

I use five different carts and use one SUT currently wired at 1:10.

@theflattire If you hear differences between the cartridges its likely got more to do with loading the transformer correctly than anything else.

@adam8179 That method I mentioned is daunting! Jensen Transformers did a lot of this work and so they have a pdf file you have to ask about that shows the loading their transformers are expecting, depending on the cartridge  used.

Plan B: place a 100K variable resistance across the output of the transformer while its plugged into your phono section. Find an LP of which you also have a CD or digital source. Play the CD or file. Compare to the LP and vary the control until the two sound very similar. Its likely that the LP won't sound as bright... At any rate disconnect the potentiometer and measure its value using a Digital Voltmeter on the Ohms scale and find a fixed resistor of the same value or very close.

Right here is actually the biggest weakness of LP reproduction as opposed to digital; not noise floor, dynamic range, distortion or any of that. Its setup and that is likely a huge part of why digiphiles prefer digital; they just really haven't heard an LP set up properly and no shame in that.